1. Field of the Invention
The present invention relates to a semiconductor device using a thin film transistor (hereinafter referred to as a TFT) wherein a semiconductor film formed on a substrate and having a crystal structure is used; and a process for manufacturing the same. The semiconductor device in the present specification means any device which functions by the use of a semiconductor characteristic, and the category of the semiconductor device manufactured by the present invention includes a display device, a typical example of which is a liquid crystal display device having therein TFT""s, and a semiconductor integrated circuit (such as a microprocessor, a signal processing circuit, or a high frequency circuit).
2. Description of Related Art
In various semiconductor devices having therein semiconductor elements, such as a television, a personal computer or a portable telephone, a display for displaying characters or images is an essential means from which people recognize information. Particularly in recent years, a planar display (flat panel display), a typical example of which is a liquid crystal display device using the electro-optic property of liquid crystal, has been actively used.
As one form of the flat panel display, there is known an active matrix driving system wherein a TFT is fitted to each pixel and a picture is displayed by writing data signals successively. TFT""s are essential elements for realizing the active matrix driving system.
In almost all cases, TFT""s are manufactured using amorphous silicon. However, TFT""s have low electric field effect mobility and cannot be operated by frequencies necessary for processing picture signals. Therefore, TFT""s are used only as switching elements fitted to respective pixels. A data line driving circuit for outputting picture signals to data lines or a scanning line driving circuit for outputting scanning signals to scanning lines is processed by an outside IC (IC driver) mounted by TAB (tape automated bonding) or COG (chip on glass).
However, as the density of pixels becomes larger, the pitch of the pixels becomes narrower; therefore, it is considered that the system in which a driver IC is mounted has a limit. For example, in the case that UXGA (pixel number: 1200xc3x971600) is supposed, 6000 connecting terminals are required in an RGB coloring system at the simplest estimate. An increase in the number of the connecting terminals causes an increase in the probability that contact faults are generated. Moreover, the peripheral area (frame area) of its pixel section increases. As a result, it is unsuccessful that a semiconductor device using this as a display is made small-sized, and the design of the appearance thereof is damaged. In light of such background, a display device integrated with a driving circuit is clearly demanded. By integrating a pixel section with scanning line and data line driving circuits on a single substrate, the number of connecting terminals decreases drastically and the area of the frame area can also be made small.
As a means for realizing the above, suggested is a method of making TFT""s of a polycrystal silicon film. However, even if polycrystal silicon is used to form TFT""s, after all, the electric properties thereof are not equivalent to the properties of MOS transistors formed in a monocrystal silicon substrate. For example, the electric field effect mobility thereof is not more than {fraction (1/10)} of that of monocrystal silicon. This method has a problem that off-state current is large because of defects formed in boundaries between crystal grains.
However, for a data line driving circuit, high driving ability (on-state current, Ion) is required and an improvement in the reliability thereof is also required by preventing deterioration based on hot carrier effect. In addition, for a pixel section, low off-state current (Ioff) is required.
As a TFT structure for reducing the off-state current, a lightly doped drain (LDD) structure is known. This structure is a structure wherein an LDD region, to which an impurity element is added at a low concentration, is arranged between a channel formed region and a source region or a drain region formed by the addition of an impurity element at a high concentration. As a structure effective for preventing deterioration of the on-state current based on hot carries, there is known an LDD structure wherein a gate electrode is overlapped with some portions of an LDD area (hereinafter, the gate-drain overlapped LDD is abbreviated to the GOLD).
A TFT is manufactured by layering semiconductor films or conductive films while etching the films into predetermined shapes using photomasks. However, if the structure of the TFT is optimized to obtain characteristics required for a pixel section or each driving circuit, the number of photomasks increases, the manufacturing process is complicated and the number of steps disadvantageously, inevitably increases.
It is an object of the present invention to provide a technique capable of improving the characteristic of a TFT and realizing a TFT of an optimum structure for the driving conditions of a pixel section or a driving circuit using a small number of photomasks.
To obtain the above object, a thin film transistor of a semiconductor device according to the present invention includes a semiconductor film, a first electrode, and a first insulating film put between the semiconductor film and the first electrode, and also includes a second electrode, and a second insulating film put between the semiconductor film and the second electrode. The first electrode and the second electrode are overlapped with each other, with a channel formation region of the semiconductor film put between them.
In addition, according to the present invention, in the case of a TFT in which the decrease of OFF current is regarded more important than the increase of ON current, e.g., a TFT which is formed as a switching element in the pixel section of the semiconductor device, a constant voltage (common voltage) is applied to the first electrode. This constant voltage is set lower than threshold voltage in the case of an n-channel type TFT and set higher than threshold voltage in the case of a p-channel type TFT.
By applying the common voltage to the first electrode, it is possible to suppress threshold irregularity and to suppress OFF current compared with the TFT which includes only one electrode.
Further, according to the present invention, in the case of a TFT in which the increase of ON current is regarded more important than the decrease of ON current, e.g., a TFT which is included in the buffer or the like of the driving circuit of the semiconductor device, the same voltage is applied to the first and second electrodes.
In the specification, the driving circuit means a circuit which generates signals for displaying images on a pixel section. A data line driving circuit and a scanning line driving circuit are, therefore, driving circuits.
By applying the same voltage to the first and second electrodes, the spread of a depleted layer is accelerated substantially as in the case of making the semiconductor film thin and it is, therefore, possible to lower the sub-threshold coefficient of the TFT and to improve the field effect mobility of the TFT. Accordingly, compared with a TFT which includes only one electrode, ON current can be increased. It is thereby possible to decrease driving voltage by using the TFT having this structure in the driving circuit. In addition, since ON current can be increased, the TFT can be made small in size (the channel width thereof can be particularly made small). It is thereby possible to improve the integration density of the TFT. Besides, it is possible to suppress interface scattering and to increase transconductance (gm).
The circuit diagram of the thin film transistor of the present invention will be described with reference to FIGS. 31A, 31B and 31C. In FIGS. 31A, 31B and 31C, only p-channel type TFT is typically shown. An n-channel type TFT is opposite in arrow direction to the p-channel type TFT. FIG. 31A is a circuit diagram of an ordinary TFT which includes only one electrode. FIG. 31B is a circuit diagram of a TFT according to the present invention wherein two electrodes are provided with a semiconductor film put therebetween, and a constant voltage (ground voltage in this case) is applied to one of the two electrodes. FIG. 31C is a circuit diagram of a TFT according to the present invention wherein two electrodes are provided with a semiconductor film put therebetween and the two electrodes are electrically connected to each other. In this specification, the present invention will be described with reference to the circuit diagrams shown in FIGS. 31A, 31B and 31C.